Transcription of the late genes of bacteriophage P2 depends on the P2 DNA replication genes, the P2 ogr gene product, and the host RNA polymerase. Satellite phage P4 can activate P2 late gene expression in a strain lysogenic for P2 by causing derepression of the helper prophage. P4 can also activate late gene expression when the normal P2 pathway is blocked by mutation in one of the DNA replication genes. This transactivation depends on the P4 Delta gene and initiates transcription at the same sites as nornmal P2 late gene expression. We intend to define those features of P2 late promoters that are required for recognition by the host RNA polymerase in the presence of phage-encoded regulatory proteins. The functional domains of the late promoters will be defined by BAL-31 deletion and in vitro mutagenesis. The P2 ogr protein will be purified and characterized. We will recreate P2 late gene transcription in a purified, reconstituted in vitro reaction in order to identify additional components that may be involved in the modulation of promoter specificity. Interactions between the transcriptional regulatory proteins and the DNA template will be probed by kinetic and chemical methods. The P2-P4 system provides an excellent model for studying mechanisms by which the host transcriptional machinery can recognize new promoters in the presence of phage-encoded factors. In addition, the ability of P4 to turn on expression of latent prophage helper genes is in some sense analogous to transformation by oncogenic viruses, which involves alterations in host genome transcription.